Answers

I saw a documentary about a call centre in South Africa where they spent 3 months trying to get people to speak with a British accent so that British customers could easily understand them. Unfortunately there was hardly any improvement. Some foreign students of English have very good accents but few are perfect.

If he wants to speak as you do, just keep going. He'll pick it up automatically.
I taught a German woman English for a while. After about 3 months she told me she'd been at a dinner party the night before, where the person beside her had asked why she had a Scottish accent. Obviously she had picked it up from me, although my own accent isn't strong.

It depends which 'British' accent you want him to learn.
Many people can learn another language and master the correct accent but it depends on their natural aptitude and the competence of the teacher.

If he is in England, he will eventually pick up an English accent. But yeah if you have the patience, it would be possible. They have to notice themselves, the difference in the accents, otherwise it will never work.